Let's say that a small group (around 20 or so) of well-equipped, highly intelligent ETs have secretly made their way on to Earth. They have all the knowledge they need of our various languages and cultures to fit in, along with disguise technology so they can assume the identity of anyone.
Now let's add that they are also superhumanly skilled at trading and market manipulation, and have made it their goal to cause as much turmoil in as many economies as possible without getting their hands dirty (directly anyway).
Assuming a generous starting capital from having assimilated the identities of various wealthy private entities, how much can they influence the markets? Can they actually irreversably damage the global economy?